here you go.... mercury rates their motors hp's at the prop, in other words you are getting about 540 at the crank and 525 at the prop with a mercury 525 if not more.
ilmor rates strictly at the crank so you are getting 625 at the crank and around 605 at the prop on a good motor.
Weight of the boat makes a big difference as well. Mercury can handle the weight on heavy boats a lot better then an ilmor, figure an ilmor as a small block 350 with 2 more cylinders bolted onto the back. Components in a ilmor are much easier to stress due to there smaller size hench the weight difference between a ilmor engine and a mercury motor. Sure ilmor has an aluminum block compared to a cast iron like a mercury, but the ilmor is a much longer block due to the extra cylinders. Mercury uses much bigger components inside the block, this is where most of its weight comes from.
ilmor motors are ran on a much less conservative setting than mercury. in other words you can turn a mercury motor up (advance its ignition, add a blower) and get more power where a ilmor there are no upgrades available. Mercury sends their stock motors out on a very conservative setting.
just purchased a new boat myself, the ilmor 710 did about 93 and the mercury 700 (what i went with) does 100. The mercury 700sci weighs about 300 pounds more than the ilmor 710 and still out performed it.
Maintenance is much easier, drive options are better (you can now order the 525 with an NXT drive, which i highly recommend if in your price range).
don’t get me wrong the ilmor will probably out perform the mercury due to the horsepower difference but not by as much as you would think.
sorry for the spelling mistakes, in a hurry..